Exchange server 2010

I m new to exchange environment..Can I know how to create web application outlook in exchange 2010 because it shows error mesage when I wan to configure the web app outlook. it shows the old exchange 2010 must be removed first.. SInce it has some AD roles I am very worried to remove the old exchange...help me pls

Can you elaborate your exact issue which u r facing & r u co-existed Ex2010 or created a new Ex2010 ORg to smoothen the troubleshooting.

First of all when you add Ex2010 Server (with all the roles). By default it will create the Outlook Web App for local & for external u need to specify URL for OWebApp.
After you need to enable or disable OWebApp for users in EMC.